Radiation dosimetry involves the quantitative measurement of the physical changes that occur in matter upon exposure to ionizing radiation such as beta and alpha particles, neutral particles such as neutrons, and electromagnetic radiation such as X-rays and gamma rays. It is an important aspect of numerous civilian and military applications, including individual and environmental monitoring, retrospective and accident dosimetry, radiation therapy dosimetry, diagnostic radiology and nuclear medicine dosimetry, and neutron, particle and space dosimetry.
Thermoluminescence dosimetry is one of the most widely used and cost-effective techniques for radiation dosimetry, and has been extensively studied, both experimentally and theoretically.
The radiation-sensitive element of a thermoluminescent dosimeter (TLD) is a small quantity, typically less than 100 mg, of an inorganic crystal doped with metal impurities known in the art as “activators.” The activators provide the crystal with the energy storage capacity as well as the luminescent properties that are required for the crystal to function as a thermoluminescent phosphor upon exposure to ionizing radiation. As generally understood, the activators provide point defects, known as traps and luminescence centers, in the crystal lattice of the thermoluminescent phosphor. When the phosphor is exposed to ionizing radiation, electrons and holes are captured in metastable states near the trap centers defined by local potential energy minima until the electrons and holes are sufficiently thermally stimulated to enable them to overcome the potential energy barriers. The thermally stimulated electrons and holes can then recombine at the luminescence centers, emitting photons, referred to as thermoluminescent (TL) emission, as they do so. See B. Justus, M. Miller, and A. Huston, “Dosimetry Measurement,” The Measurement, Instrumentation and Sensors Handbook (1999), the entirety of which is hereby incorporated by reference into the present disclosure.
Peter Braunlich and others first showed that TL glow curves could be measured by heating thin layers of phosphor powder mixed with a polymer binder and spread on a glass slide. Infrared pulses at 10.6 microns from a carbon dioxide laser were focused onto the powder film and the light was absorbed by both the glass and the polymer binder, heating the phosphor and resulting in TL emission. The TL was characterized by assuming that the thin phosphor layer made no contribution to the thermal properties of the sample (sample=phosphor+binder+glass substrate). See P. Braunlich, J. Gasiot, J. P. Fillard and M. Castagne, “Laser heating of thermoluminescent dielectric layers,” Appl. Phys. Lett. 39(9), 769-771 (1981); and J. Gasiot, P. Braunlich, and J. P. Fillard, “Laser heating in thermoluminescence dosimetry,” J. Appl. Phys. 53(7), 5200-5209 (1982).
Later, Braunlich demonstrated that a number of different phosphor configurations could be effectively heated by a continuous wave (cw) CO2 laser with a Gaussian beam profile. For example, Harshaw TLD-100 chips were directly heated by the CO2 laser due to the small absorption of lithium fluoride at 10.6 microns (abs. coef.=40 cm−1). See A. Abtahi, P. Braunlich, R. Kelly, and J. Gasiot, “Laser stimulated thermoluminescence,” J. Appl. Phys. 58(4), 1626-1639 (1985). Since the 1/e attenuation length in the LiF was 250 microns, the light penetrated a significant depth into the 900 micron thick chips.
In one experiment by Braunlich, free-standing polyimide films containing TLD-100 powder were heated with the laser, while in another, thin films of TLD-100 powder in a silicone binder were coated onto glass slides. These films could be heated effectively when the laser light was incident from either the front (powder in binder) side or the back side (glass slide). However, the TL response curves varied significantly depending on the laser power and the details of the sample preparation and Braunlich was unable to accurately model the TL response of any of these samples that were stimulated with a Gaussian beam profile. See A. Abtahi, P. Braunlich, T. Haugan, and P. Kelly, “Investigation of Thermoluminescence Efficiencies at High Laser Heating Rates,” Radiation Protection Dosimetry 17, 313-316 (1986).
Braunlich next developed a general solution for the TL response of a two-layer TLD system when the dosimeter was stimulated by a uniform circular laser beam, and later showed that laser heating of the dosimeter yields TL glow curves that are similar to those obtained using conventional heating methods. See Abtahi et al. (1986), supra; see also P. Kelly, A. Abtahi, and P. Braunlich, “Laser-stimulated thermoluminescence. II,” J. Appl. Phys. 61(2), 738-747 (1987). The general solution for the temperature increase in a two-layer system was simplified by assuming that one of the two dosimeter layers was much thinner than the other. When the thermal diffusivity of both layers was assumed to be approximately the same, lateral heat diffusion in the thin layer was ignored and the thermal diffusion in the sample was assumed to be governed only by the properties of the thick layer. Only one limiting case was discussed, i.e., a thick absorbing layer with heat transport across the interface into a thin layer of phosphor. It was assumed that lateral thermal diffusion only occurred in the thick absorbing layer, while no lateral thermal diffusion occurred in the thin phosphor layer. Experiments were performed using 35-40 micron thick layers of phosphor on 150 micron thick glass slides (the absorber). However, Kelly reported that signal reproducibility was a problem due to difficulties encountered in preparing uniform thin films of the phosphor/binder mixture on the glass substrates. See Kelly et al. (1987), supra.
The laser-heated TLD (LHTLD) systems developed by Braunlich and his colleagues required the development of unique TLD badges. The design of the laser-heated TLD dosimeter elements developed by Braunlich was guided by practical consideration of two conflicting requirements: minimum layer thickness and high sensitivity. See P. Braunlich, “Present State and Future of TLD Laser Heating,” Radiation Protection Dosimetry 34, 345-351 (1990). The LHTLD system developed by Braunlich et al. functioned most effectively when the laser-heated phosphor layer was as thin as possible. However, for adequate sensitivity of the dosimeter to small radiation doses to be maintained, the mass of the phosphor could not be reduced below a minimum (the minimum specific mass in mg/mGy). In other words, the dosimeter elements in the system could not be manufactured as thin as desired because it was necessary to ensure that the sensitivity did not fall below a critical value. In order to satisfy these conflicting requirements, Braunlich developed a unique LHTLD dosimeter consisting of a fine-grain TLD powder and a silicone binder, printed onto metallized polyimide foils. Braunlich also developed dosimeters using all-inorganic elements, because the dosimeters containing polymer and other organic material exhibited spurious TL emissions and light sensitivity problems. See P. Braunlich, “Present State and Future of TLD Laser Heating,” Radiation Protection Dosimetry 34, 345-351 (1990). Page 347, column 1, paragraph 4.
Others have further examined the use of laser-heated phosphors for radiation detection.
John Lawless and D. Lo studied the laser-heated TL emission properties of phosphors as a function of the temporal profile of the laser heating, and predicted that for a wide range of laser heating profiles (linear, quadratic, or square root with respect to time) the intensity of the peak of the glow curve should follow a general scaling law (peak intensity is inversely proportional to the time at which the peak occurs). See J. Lawless and D. Lo, “Thermoluminescence for nonlinear heating profiles with application to laser heated emissions,” J. Appl. Phys. 89(11), 6145-6152 (2001). However, he found that the experimental results of Gasiot et al. (1982), supra, do not follow the general scaling law.
Ofer Gayer and Abraham Katzir described a remote laser heating technique in which a silver halide fiber was used to deliver CO2 laser light to a small point on a sample. See 0. Gayer and A. Katzir, “Laser-fiberoptic non-contact controlled heating of samples for thermoluminescence measurements,” Journ. of Lumin. 113, 151-155 (2005). The TL emitted by the sample was transmitted to a PMT by a standard silica fiber. The temperature of the heated spot was monitored using infrared radiometry. The infrared light was transmitted to an infrared radiometer using a second silver halide fiber. Katzir subsequently reported that the reproducibility observed for the TL measurements could be improved by eliminating the silver halide optical fiber. See R. Ditcovski, O. Gayer and A. Katzir, “Laser assisted thermoluminescence dosimetry using temperature controlled linear heating,” Journ. of Lumin. 130, 141-144 (2010). Measurements performed on small spots on the samples were problematic due to inhomogeneities in the samples. Katzir found that expansion of the CO2 laser beam, using a ZnSe lens and a mirror, allowed illumination of the entire sample and improved the performance of the LHTLD system. See R. Ditcovski, O. Gayer, and A. Katzir, “Laser assisted thermoluminescence dosimetry using temperature controlled linear heating,” Journ. of Lumin. 130, 141-144 (2010).